Porous materials, for example polymer foams, having pores of a size in the range of a few microns or significantly less and a high porosity of at least 70% are particularly good thermal insulators for theoretical reasons.
Such porous materials having a small average pore diameter can, for example, be in the form of organic xerogels. The term xerogel is not always used uniformly in the literature. In general, a xerogel is understood to be a porous material which has been produced by a sol-gel process and the liquid phase has been removed from the gel by drying below the critical temperature and below the critical pressure of the liquid phase (“subcritical conditions”). In contrast, the term aerogels is generally used when the removal of the liquid phase from the gel has been carried out under supercritical conditions.
In the sol-gel process, a sol is firstly prepared from a reactive organic gel precursor and the sol is then gelled by means of a crosslinking reaction to form a gel. To obtain a porous material from the gel, for example a xerogel, the liquid has to be removed. This step will hereinafter be referred to as drying for the sake of simplicity.
In the processes of the prior art, the liquid can be removed from the gel under, for example, supercritical conditions or by means of supercritical fluids, i.e. at pressures and temperatures above the critical pressure pcrit or the critical temperature Tcrit of the liquid. Such drying processes which include supercritical drying are widely known.
However, drying under supercritical conditions is very complicated in terms of apparatus since drying has to be carried out in closed vessels under defined pressure and temperature conditions. In addition, solvent changes are required. This additionally complicates the process. Drying using supercritical fluids therefore adversely affects the economics. As an alternative, the liquid can be removed by freeze drying. However, the liquid undergoes a change in volume on freezing, as a result of which the three-dimensional polymer framework is destroyed. A powder rather than a foam is obtained in this way. In addition, a freeze drying step is complicated in terms of apparatus.
If a gel is dried in a subcritical process step, then the pore structure generally changes and the gel shrinks with a simultaneous reduction in the porosity of the resulting xerogel compared to analogous removal of the solvent under supercritical conditions. The reason is the capillary forces which act during vaporization of the solvent under subcritical conditions. In addition, the capillary forces at average pore sizes of less than 1 micron are particularly strong since they increase in inverse proportion to decreasing pore size.
Known strategies for reducing the shrinkage comprise production of a sufficiently stiff or stable gel by means of specific compositions and reduction of the capillary forces, for example by replacement of the solvent by a less polar solvent which causes lower shrinkage on drying of the gel.
However, replacement of the solvent is complicated in terms of apparatus and in many cases is undesirable. In addition, the subcritical drying of the gel even after replacement of the solvent by one which causes reduced capillary forces nevertheless often leads to a reduction in the porosity.
Known organic xerogels are, for example, based on phenol-aldehyde resins or on polyurethane and/or polyurea. Processes for producing them which provide alternative measures for limiting the shrinkage during drying are likewise known per se. The known processes which do not require replacement of the solvent to be extracted are based on specific compositions which form a sufficiently stable porous material or on the use of fibrous reinforcing materials.
In addition, aerogel foam composites are known from WO-2007/146945. The aerogel foam composites mentioned comprise, in particular, inorganic aerogels which are cured in foams such as polyurethane foams, followed by a supercritical drying step. The publication mentions the possible use of organic aerogels and also the use of subcritical drying. The publication uses a foam matrix in order to reduce the inherent brittleness of the (inorganic) aerogels and at the same time improve further mechanical properties such as flexibility.
Xerogels based on polyurea or polyurethane are known per se and are often superior to inorganic aerogels in terms of the mechanical properties.
WO-2008/138978 discloses xerogels which are based on at least one polyfunctional isocyanate and at least one polyfunctional aromatic amine and whose volume-weighted average pore diameter is not more than 5 microns.
However, the thermal conductivity of the known xerogels is not sufficient for all applications. For applications in the pressure range above the vacuum range, for example in the pressure range from about 1 to about 100 mbar but in particular at atmospheric pressure, the thermal conductivity is generally not satisfactory. In addition, the materials properties, for example the mechanical stability of the xerogel, the porosity and in particular the density, are unsatisfactory.
Subcritical removal of the solvent is extremely desirable from the point of view of economical production. Thus, the xerogels are in this respect superior to the aerogels. The present invention relates exclusively to xerogels. Subcritical drying is generally ruled out for inorganic xerogels because of their poor mechanical properties.